Successful fertilization in mammals requires the union of the sperm and egg nuclei within the activated oocyte cytoplasm. Cytoskeletal assembly within the oocyte is required for sperm penetration, pronuclear decondensation and apposition as well as chromosome segregation during meiosis and mitosis. There is limited information on these central events in mammalian reproduction but studies with a few species suggest that different mammals may accomplish fertilization by varying means. Consequently, it is unclear if extrapolating the mechanisms involved in any one mammalian species accurately reflects the events in another mammal, including humans. Our research is therefore aimed at understanding the process of genomic union during fertilization in various marsupial and eutherian mammals, a key step in understanding the reproductive process. We will be using the FESEM to visualize sperm on the surface of and hopefully entering the oocyte. We will also be using the deep-sectioning capabilities of the multiphoton microscope to examine the cytoarchitecture of the bovine egg at various stages after fertilization.